Graphite films are characterized by lower tearing strength, as compared to typical plastic films and other films. Graphite films are classified into: graphite films (also referred to as “natural graphite sheets”) produced by an expanding method; and graphite films (also referred to as “fired polymer-derived graphite film”) produced by polymer thermal decomposition.
Most of the natural graphite sheets have tearing strength (average tearing force) of about 0.1 N. This is because typical natural graphite sheets have molecular structures in which the degree to which molecules of the graphite film are uniformly oriented in parallel to a planar direction of the film is not high. On the other hand, in the case of the fired polymer-derived graphite films, their molecular structures are such that the molecules of graphite are more uniformly oriented in parallel to its planar direction with film designing intended to enhance the performance of the graphite film in its planar direction. Therefore, the fired polymer-derived graphite films cannot resist shearing stress and thus have the tendency to show low tearing strength (average tearing force).
Graphite films can be processed with self-adhesive material layers, insulating films, other protective films, etc. into composite products. In such a stage of processing, graphite films having average tear strength suffer from defects such as splitting and winding deviation.
That is, graphite films have the problem that the occurrence of the defects in the stage of processing increasingly increases along with sophistication of the graphite films. In particular, this problem was serious for graphite films having average tearing force of not greater than 0.08 N.
A method for a reinforcing natural graphite sheet is disclosed in Patent Literature 1. According to this method, a natural graphite sheet and a thin film of thermoplastic polymer having a given softening temperature are laminated to each other as they are supplied between temperature controlled two calender rolls by a continuous method.
However, application of the technique disclosed in Patent Literature 1 to graphite films having average tearing force of not greater than 0.08 N causes the following problem. That is, the fired polymer-derived graphite film is split at the edges thereof, as shown in FIG. 18, before supplied between the two rolls. This results in the failure to laminate.
Further, a method for reinforcing a sheet-type, fired polymer-derived graphite film (This means a fired polymer-derived graphite film produced from polymer films in sheet form as a raw material. This term is used to distinguish from the fired polymer-derived graphite film produced from a roll of long polymer film as a raw material) is disclosed in Patent Literature 2. According to this method, the sheet-type, fired polymer-derived graphite film is reinforced by lamination to a sheet having a self-adhesive material by means of a laminator. However, application of the technique disclosed in Patent Literature 2 to graphite films having average tearing force of not greater than 0.08 N and having a length of not less than 1000 mm causes the following problem. That is, the fired polymer-derived graphite film is split at the edges thereof, as shown in FIG. 18, before supplied between the two rolls. This results in the failure to laminate.
For such an extremely weak graphite film showing an average tearing force of not greater than 0.08, it was a serious problem that defects (for example, occurrence of splitting, winding deviation, wrinkling, etc.) are caused in the processing step.